Precast concrete structural framework systems are known that comprise some or various of the three types of basic elements that compose the system described hereafter: support columns, with or without capital; beams supported on the columns; and slab segments supported on beams or directly on the columns.
There are many variants among the multiple framework systems of this kind. For a better understanding of which aspects differ from some variants to others, nine essential features of this kind of structure are established:                A) Existence of a CAPITAL on top of the column.        B) Geometry of the BEAMS, namely, of the elements or parts of the floor that are directly supported on the columns.        C) Section of the SLAB, or of the part of the floor that is not directly supported on the columns, but it does on the beams.        D) SIZE and SHAPE of the precast elements.        E) Type of REINFORCEMENT used in the floor, namely in the beams and slabs.        F) ONE-WAY or TWO-WAY direction of the floor and, in particular, of the slab.        G) Solution for the SUPPORT of precast elements on others, especially during the assembling process.        H) Solution for the CONTACT in the zone of support between one element of precast concrete and its support, especially during the assembling process.        I) Solution to grout or not the JOINTS between elements.        
A) Capital
Examples of this kind of structural frameworks in which the columns do not have capitals are described in GB1084094 (Zezelj), U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,553,923 (Dompas) and U.S. Pat. No. 5,504,124 (Tadros).
Examples of this kind of structural frameworks in which the columns do have capitals are described in U.S. Pat. No. 2,776,441 (Dobell), U.S. Pat. No. 3,918,222 (Bahramian) and U.S. Pat. No. 8,011,147 (Hanlon). In the latter case the capital is not quadrangular.
B) Beams
An example of this kind of structural frameworks in which the beams are narrow and protrude under the slab is described in U.S. Pat. No. 2,776,441 (Dobell).
Examples of this kind of structural frameworks in which the beams are wide but protrude under the slab are described in U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,918,222 (Bahramian), U.S. Pat. No. 5,504,124 (Tadros) and U.S. Pat. No. 8,011,147 (Hanlon).
Examples of this kind of structural frameworks in which the beams are wide and flat or indistinct to the ensemble of the slab are described in GB1084094 (Zezelj) and U.S. Pat. No. 3,553,923 (Dompas).
C) Slab
An example of this kind of structural frameworks in which the slabs are solid is described in U.S. Pat. No. 3,495,341 (Mitchell Jr)
Examples of this kind of structural frameworks in which the slabs have a T-section are described in GB1084094 (Zezelj), U.S. Pat. No. 3,918,222 (Bahramian) and U.S. Pat. No. 8,011,147 (Hanlon).
Examples of this kind of structural frameworks in which the slabs have double-T or box section are described in U.S. Pat. No. 2,776,441 (Dobell), U.S. Pat. No. 3,553,923 (Dompas) and U.S. Pat. No. 5,504,124 (Tadros).
D) Size and Shape
Examples of this kind of structural frameworks in which the precast elements are small blocks or segments are described in U.S. Pat. No. 2,776,441 (Dobell) and U.S. Pat. No. 3,553,923 (Dompas).
An example of this kind of structural frameworks in which the elements are flat and of big dimensions is described in GB1084094 (Zezelj).
Examples of this kind of structural frameworks in which the elements are long and of big dimensions are described in U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,918,222 (Bahramian), U.S. Pat. No. 5,504,124 (Tadros) and U.S. Pat. No. 8,011,147 (Hanlon).
E) Reinforcement
Examples of this kind of structural frameworks with passive reinforcement are described in U.S. Pat. No. 3,495,341 (Mitchell Jr) and U.S. Pat. No. 5,504,124 (Tadros).
An example of this kind of structural frameworks with pretensioned reinforcement is described in U.S. Pat. No. 8,011,147 (Hanlon).
Examples of this kind of structural frameworks with post-tensioned reinforcement are described in U.S. Pat. No. 2,776,441 (Dobell), GB1084094 (Zezelj), U.S. Pat. No. 3,553,923 (Dompas) and U.S. Pat. No. 3,918,222 (Bahramian).
F) One-Way or Two-Way Flexure
Examples of this kind of structural frameworks made of elements with flexure in one direction (one-way flexure) are described in U.S. Pat. No. 2,776,441 (Dobell), U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 5,504,124 (Tadros) and U.S. Pat. No. 8,011,147 (Hanlon).
Examples of this kind of structural frameworks made of elements with flexure in two directions (two-way flexure) are described in GB1084094 (Zezelj), U.S. Pat. No. 3,553,923 (Dompas) and U.S. Pat. No. 3,918,222 (Bahramian).
G) Support
Concerning the way in which the elements are supported on others, at least three families can be distinguished: the ones that use supports on metal parts, provisional or definitive; the ones that use half lap supports in concrete; and ones that use more or less complex geometries of interlocking between elements, by means of metallic or concrete edges, which are able to transmit non-vertical efforts.
Examples of this kind of structural frameworks with joints that use supports on metal parts are described in GB1084094 (Zezelj) and U.S. Pat. No. 8,011,147 (Hanlon).
The precast structural frameworks with joints that use half lap supports in concrete are much more common. Among the structural frameworks of the type studied here, examples of half lap support are described in U.S. Pat. No. 2,776,441 (Dobell), U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,918,222 (Bahramian) and U.S. Pat. No. 5,504,124 (Tadros).
Examples of joints that use interlocking complex geometries between elements that can transmit non-vertical efforts are described in US409893 (Wrey), US24114438 (Henderson), U.S. Pat. No. 2,618,146 (Ciralini), U.S. Pat. No. 2,966,009 (Koch) and others. In general all these examples are not very prevailing, typical of a solution that has been proven to be more appropriate to other materials (wood, metals, etc.) than to concrete.
H) Contact
Concerning the contact between the elements of precast concrete and the element on which they are supported, the four following solutions can be differentiated: butt joint (direct support) on concrete; butt joint (direct support) on steel; on a base of mortar; and on a base of elastomer.
Examples of this kind of structural frameworks with butt joint contact between concrete and concrete are described in U.S. Pat. No. 2,776,441 (Dobell), U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,553,923 (Dompas), U.S. Pat. No. 3,918,222 (Bahramian) and U.S. Pat. No. 5,504,124 (Tadros).
Examples of this kind of structural frameworks with butt joint contact between concrete and steel are described in GB1084094 (Zezelj) and U.S. Pat. No. 8,011,147 (Hanlon). The solutions with mortar or with elastomer are more common for the support of precast elements on elements executed on site.
I) Joints
Regarding the way in which the gaps between elements in the joints are resolved, at least three families can be differentiated: the one using an open joint that gets grouted with mortar or concrete; those in which the joints present gaps quite small but imperfect, which do not get grouted; and those in which the joint allows such a perfect interlocking, whether metallic or made of concrete, that it does not require grouting.
An example of this kind of structural framework with joints that use open joint that gets grouted with mortar or concrete is described in GB1084094 (Zezelj).
Examples of this kind of structural frameworks with joints that use joints with small gaps small that do not get grouted are described in U.S. Pat. No. 3,495,341 (Mitchell Jr), U.S. Pat. No. 3,918,222 (Bahramian), U.S. Pat. No. 5,504,124 (Tadros) and U.S. Pat. No. 8,011,147 (Hanlon).
Examples of joints with perfect interlocking without concrete grouting are described in US409893 (Wrey), U.S. Pat. No. 2,618,146 (Ciralini), U.S. Pat. No. 2,966,009 (Koch) and others. In general all these are not very prevailing examples, typical of a solution that has been proven to be more appropriate to other materials (wood, metals, etc.) than to concrete.
Each of the variants of the nine essential features previously described for the design of structural systems has its limitations. These are described hereafter following the same criteria of the nine essential features.
A) Capital
The systems that do not use capitals have, as their major problem, strong efforts in the joints of the column and the elements it bears. Furthermore, being these joints of small dimensions, they turn out to be almost impossible to reinforce. This often limits a lot the ability of this kind of frameworks to absorb efforts of negative flexure, and also very significantly reduces the shear strength of the joints. That is why this type of systems often has limited spans and limited maximal loads. However, the systems using columns with solidary capitals do not have these strength and stiffness limitations in negative flexure zones. Despite this, they entail a logistic problem due to their irregular geometry.
B) Beams
The systems that use narrow (downdropping) beams, pose the obvious problem of the lack of flatness of the floors, leading to functional issues. Nevertheless they are much more efficient in structural terms than wide (nondropping) beams.
The wide (nondropping) beams are in the opposite situation, they have less structural efficiency, but offer flat floors. The lack of structural efficiency of this kind of beams, in many cases can be compensated increasing the width of the beam and/or its reinforcement. In the case of prestressed beams, and even more in those having post-tensioned reinforcement, this lack of efficiency, may also be compensated by increasing the prestress load, which means increasing the amount of reinforcement. Wide beams that drop under the slab are in a transition situation between the narrow (downdropping) beam and the wide (nondropping) beam.
C) Slab
The structural systems with solid slab have the limitation of a worse sitffness/weight ratio than slabs with voided sections. Despite this, the solid slabs have a significantly greater shear strength than voided sections (T sections or double T sections). That is why they are suitable for elements under significant loads.
The systems with T section slabs have a better stiffness/weight ratio—this is particularly true under positive moments. However, they have a quite lower performance under negative moments.
Those systems with double T section (or box section) slabs have the best sitffness/weight ratio both under positive and negative moments. Furthermore, these sections are much more suitable to prestressing than T sections, because they can stand greater prestress loads at transfer. In any case their biggest inconvenient is that they often need concrete being poured in 2 phases.
D) Size and Shape
Those structural systems that use small precast elements or segments may have some advantages on a logistic level, but have an important limitation during the erection: they need formwork and shoring. Furthermore, in systems of this type the amount of joints is very high and often distributed all over the floor, so they may include more weak points on a level of strength, of stiffness and/or of durability. One last limitation is that the flat floors formed by small elements are only possible using prestressing, and normally by means of post-tensioned reinforcement, since the assembling of the elements is done on site.
On an opposite situation, we may find those systems including big flat elements. This type of elements have many advantages: they do not need shoring or formwork on site, because precast elements they are directly beared on the definitive supports; they can be precast with reinforcement in two directions and therefore they are very suitable to work two-way; and they reduce to the minimum the number of joints between elements and these can be studied so that they can be located in lesser critical zones. Despite their big advantages, they have an important disadvantage, almost insurmountable: their weight and mainly their big dimensions often imply important logistic limitations. This ends up limiting by the maximum size of the elements.
The systems of big and long elements are a compromise solution between the two previous systems, and they gather the advantages of both systems, in order to minimize their disadvantages. Long elements can be designed to be directly beared on their definitive supports, avoiding shoring and props. Because of their geometry, they are not specially suited to be reinforced at the factory with two-way reinforcement. Despite this, on site post-tensioning may achieve these elements to work two-way.
E) Reinforcement
Systems that use passive reinforcement or pretensioned reinforcement but not post-tensioned reinforcement, have two very significant limitations: on one hand, joints are discontinuity points; and on the other, pretensioned reinforcement can only be placed one-way. Passive reinforcement is often one-way, even if there is not much reason for that.
Those systems with passive or pretensioned reinforcement where concrete is not poured on site, or pouring is limited to the joints, as the ones described herein, it is not common to have reinforcement passing through the joints. Furthermore, the concrete poured in the joints has no capacity to resist tensions. In some cases, the existence of non-grouted small gaps between one element and another even prevent the joints to bear compressions. This implies big discontinuities regarding efforts, and an inefficient structural utilization of the elements, resulting in greater needs both of reinforcement and of depth to satisfy strength and stiffness requirements. Furthermore, resulting structures have very low structural redundancy, which means a bigger risk of chain collapse.
In contrast, those systems including passive or pretensioned reinforcement where considerable amounts of concrete is placed on the site have clearly less problems associated to joints discontinuities. The greater the portion of cast in situ concrete, the lesser problems with joints, both between precast elements and between precasts and cast in situ concrete. These type of solutions have among their greater advantages the chance to add passive reinforcement placed in the site, for example in zones under negative moments. Despite the advantages offered by the on site pouring of a part of the concrete, this type of structures keep having at least two limitations: on the one hand execution speed is limited by the need to wait for concrete hardening; and on the other hand pretensioned reinforcement is not continuous from one bay to the other.
Most of the above descrived limitations (joints, one-way flexure) may be solved in those systems using post-tensioned reinforcement.
Post-tensioning the reinforcement offers, in general, significant advantages over the use of passive reinforcement or pretensioned reinforcement. Among these, the possibility of prestressing concrete both under positive and negative flexures stands out; the possibility to erect prestressed cantilevers; and the possibility to close joints between elements which concrete has been poured in different moments. Despite this, using post-tensioned reinforcement in precast poses the two difficulties: on the one hand, threading the reinforcement through the several elements, which must be provided with holes or recesses properly facing each other; and on the other hand, an proper erection process must be foreseen that enables the protection of post-tensioning reinforcement to avoid its rusting must be foreseen, such as injecting the ducts or any other equivalent process. Only GB1084094 (Zezelj) gives a technically feasible solution to these two problems, consisting in placing the bare post-tensioned reinforcement inside an open joint that is subsequently concreted. Despite properly solving the two mentioned problems, a limitation may be pointed out: the layout of the tendons is polygonal. As known, this is a type of layout that is very effective, even if the parabolical tayout is known as more oprtimal. Out of the field of building construction, segmental bridges construction has efficiently solved both watertightness of joints and the problem of holes facing each other. However, this bridges construction technique cannot directly be used in buildings.
F) One-Way or Two-Way Flexure
Those floors made of precast elements that are only capable of developing flexure in a direction (one-way flexure) have significant functional and architectural limitations. The four most important limiations of these floors are described next: a) it is impossible to palce holes (for stair cases, for instance) having its long dimension perpendicular to the flexure direction of the one-way elements; b) placing holes with the long dimension parallel to the flexure of the elements is not free of problems either, since it may enforce using header joists, often scarcely compatible with the one-way logic of the precast elements; c) having cantilevers is often complicated, as it is only possible if one-way elements of the adjoining span ara parallel to those of the cantilever; d) it is very advisable—sometimes unavoidable—to align supports, in order to guarantee that precast elements properly match with each other.
Two-way floors cast in the site do not have any of the previous limitations. Despite this, precast two-way floors are somehow more limited than those cast on the site. Their bigger drawbacks are probably that they pose limitations to the free position of supports, and also to the formation irregular perimeters or irregular holes.
G) Support
Those systems of structural frameworks that use metal parts supports have two limitations: on the one hand, they lead to concentrations of efforts in the bearing points, which must be controlled to avoid local failure of concrete; and on the other hand, metal parts can not be used as joints sealing against leakage, because those are only small isolated metal parts.
Those systems of structural frameworks that use half lap supports have at least two limitations:
First of all, the flatness of the floors is incompatible with a good shear strength of the joints. Normally, one of the two following solutions is choosed: whether the floor is kept flat, but in junction zones elements are under important stresses due to the reduction of depth typical of half lap supports. This, in turn, may lead the overdimensioning of the elements of the junction and/or to important concentrations of reinforcement in these zones. Whether the depth of at least one of the two elements is kept, on the condition that the other will have a greater depth. Of course, the least makes impossible a flat floor.
Secondly, in the event of half lap joints between two elements having both the same depth, it is improbable that the zone of the junction will have the same or a greater stiffness than the elements it joins. This is due to the fact that it is impossible to grout the bottom half of the joint with concrete to restore the stiffness.
Those structural systems that use bearing methods by means of complex geometries of interlocking between elements that can transmit non vertical efforts are not common currently in building precasting, doubtlessly due to their added difficulties in the factory production and the assembly process on site. Despite these problems, nowadays it is usual to fabricate elements with interlocking geometries of a certain geometric complexity, for segmental bridges.
H) Contact
Those systems of structural frameworks that use butt joints bearings, for concrete-to-concrete contact or concrete-to-steel contact, share the advantage of being very easy and fast to put in place, but in return demand smaller production and assembly tolerances. However, beyond the problem of tolerance there is a bigger one. In some types of supports, for example half lap supports, a butt joint support risks to cause that the load is not centred on the supposed bearing surface, and important stress concentrations may occur in small surfaces or in edges, possibly resulting in local failure and subsequent wide-ranging collapses.
Those systems that use supports in which a material (fresh mortar, elastomer, resins) is placed between the surfaces of the two elements in contact solve the two previously mentioned issues: the problem of fabrication and assembly tolerances, and the problem about the centering of the loads. On the other hand, mortar and resins will need workforce in the job. While the elastomer may either be installed on site, or may be included in (or sticked on) the precast element, the latter is an advantage, even if it still is a complication to the factory production process.
I) Joints
Those solutions based in open joint between elements have at least one important limitation: the separation between elements may allow grout leakage. This forces the use of rather dry mortars. Alternatively also some type of formwork may be used. Those joint solutions based in non-grouted small gaps, despite allowing a greater speed and simplicity of construction, have the obvious limitation of causing zones of stiffness discontinuity. This often leads to a noteworthy loss of the structure efficiency along with a reduction of structural redundancy.
Those joint solutions based in a perfect interlocking between elements are hard to implement because the fabrication and assembling tolerances common to concrete elements make it almost impossible in practice to have a totally perfect interlocking. On the other hand, a perfect interlocking of elements often seriously conditions the assembly process. That is why, despite the fact that decades ago this type of solutions appeared in numerous patents, nowadays it is a solution practically non-existent in building construction. Despite this, segmental bridges do use this type of joint, which is linked to both a peculiar production system and an erection process very characteristic of this sort of bridges.